In lithographic techniques, exposure apparatus for transferring a fine circuit pattern to a wafer to produce an integrated circuit have been used extensively. With the trend toward higher degrees of integration, higher speeds, and higher functions in integrated circuits, the scale down of integrated circuits proceeds. Exposure apparatus are required to form a circuit pattern image having high resolution on a wafer surface at a large focal depth, and wavelength reduction in exposure light proceeds. Besides g-line (wavelength, 436 nm), i-line (wavelength, 365 nm), and KrF excimer lasers (wavelength, 248 nm), which have been used as light sources, ArF excimer lasers (wavelength, 193 nm) are coming to be employed as light sources having an even shorter wavelength. Furthermore, use of an F2 laser (wavelength, 157 nm) for coping with next-generation integrated circuits having a circuit line width of 100 nm or small is expected to be effective. However, even this technique is considered to cope with only up to the generation having a line width of 70 nm.
Under these technical circumstances, a lithographic technique employing EUV light (extreme ultraviolet light) is attracting attention as a next-generation exposure light applicable to the 45-nm and succeeding generations. The term “EUV light” means a light having a wavelength in the soft X-ray region or vacuum ultraviolet region, specifically a light having a wavelength of about 0.2-100 nm. At present, use of a lithographic light source of 13.5 nm is being investigated. The exposure principle of the EUV lithography (hereinafter abbreviated as “EUVL”) is equal to that of conventional lithography in that a mask pattern is transferred with an optical projection system. However, a refractive optical system cannot be used because there is no material which is light-transmitting in the EUV light energy region, and a reflective optical system should be used (see patent document 1).
The mask for use in EUVL is constituted basically of (1) a substrate, (2) a reflective multilayer film formed on the substrate, and (3) an absorber layer formed on the reflective multilayer film. As the reflective multilayer film used is a film having a structure composed of two or more materials which differ in refractive index at the wavelength of the exposure light and are periodically superposed on the order of nanometer. Typical known materials are molybdenum and silicon. For the absorber layer, use of tantalum and chromium is being investigated. With respect to the substrate, the material thereof is required to have a low coefficient of thermal expansion so as not to deform even upon irradiation with EUV light, and use of a glass or crystallized glass having a low coefficient of thermal expansion is being investigated. The substrate is produced by highly accurately polishing a raw material which is such a glass or crystallized glass and washing the polished material.
Methods for polishing a substrate for magnetic recording media, a substrate for semiconductors, or the like to impart high smoothness are generally known. For example, patent document 2 discloses the finish polishing of a memory hard disk and the polishing of a substrate for semiconductor elements. Specifically, it discloses a polishing method for obtaining a polished work which has reduced surface roughness and in which surface defects such as minute projections (small bumps and small particles) and polishing damages (pits and scratches) have been diminished. This method comprises polishing a raw substrate with a polishing liquid composition which comprises water, an abrasive material, and an acid compound and has a pH in an acid region and an abrasive material concentration lower than 10% by weight. That patent document enumerates aluminum oxide, silica, cerium oxide, zirconium oxide, and the like as examples of the abrasive material and nitric acid, sulfuric acid, hydrochloric acid, organic acids, and the like as examples of an acid for use in pH adjustment to a value in an acid region.
Furthermore, patent document 3 discloses a glass substrate for mask blank which has a surface inhibited from having minute protruded surface defects and a polishing method for producing this substrate. In this glass substrate, the height of the protruded surface defects has been regulated to such a value (e.g., smaller than 2 nm) that the defects do not cause phase defects when a mask for exposure produced from this glass substrate is used. The polishing method comprises mirror-polishing a main surface of a glass substrate with a polishing pad while supplying a silica-containing slurry, and the slurry to be used is one from which coarse particles having a particle diameter of 1,000 nm or larger formed by silica aggregation have been removed. There is a statement in that patent document that the slurry is preferably regulated so as to be alkaline.
Patent Document 1: JP-T-2003-505891
Patent Document 2: JP-A-2003-211351
Patent Document 3: JP-A-2005-275388
However, according to the polishing method disclosed in patent document 2, when silica particles are used as the abrasive material, the particle diameter thereof is regulated so as to be in the wide range of 1-600 nm, especially preferably in the range of 20-200 nm, in order to improve the rate of polishing. From the standpoints of the diminution of minute projections and profitability, the concentration of silica particles is regulated to below 10% by weight, most preferably 7% by weight or lower. Namely, it is thought that in patent document 2, the concentration of silica particles is reduced to that level because an increase in silica particle concentration results in an increase in the amount of minute projections and that the diameter of the silica particles is regulated to 1-600 nm for compensating for the reduced concentration to thereby obtain a desired polishing rate. As a result, the surface smoothness of the substrate for magnetic-disk use obtained through polishing with this abrasive material is limited although minute projections have been diminished. Specifically, the surface roughnesses (Ra) in the Examples are 0.2-0.3 nm. To sum up, the degree of polishing obtainable by the polishing method disclosed in patent document 2 is as low as about 0.2-0.3 nm in terms of surface roughness (Ra).
Such a glass substrate having a surface smoothness of 0.2-0.3 nm in terms of surface roughness (Ra) is difficult to use as a glass substrate for reflective masks for use in EUVL, in particular, as a glass substrate required to have exceedingly high surface accuracy and smoothness as in reflective masks for use in the optical system of an exposure apparatus for producing semiconductor devices of the 45-nm and succeeding generations.
On the other hand, in the glass substrate disclosed in patent document 3, the convex defects have a specific height. However, when a main surface of a glass substrate is polished with a polishing pad while supplying a silica slurry, concave defects also are formed in the polished main surface due to fine silica particles and minute foreign matters. In patent document 3, the polishing technique is intended to diminish only the convex defects which are apt to develop because the slurry has been regulated so as to be alkaline from the standpoint of silica stability. However, as long as polishing is conducted with a slurry containing silica, it is difficult to inhibit the development of concave defects. In a glass substrate for mask blank, such concave defects having a depth exceeding a given value can be a cause of phase defects, like convex defects, when a mask for exposure produced from this glass substrate is used.
Besides the height and depth of such convex and concave defects developed on the main surface of a glass substrate, the sizes of the defects in the plane directions (areas) can be a serious problem when the substrate is used as a mask for exposure. The influence thereof becomes larger as the wavelength of the exposure light becomes shorter. The sizes of convex defects and concave defects correlate in some degree with the height of the convex defects and the depth of the concave defects. However, since such defects have various shapes, the sizes thereof cannot be unconditionally determined. This is true especially in concave defects, which differ from convex defects in cause of development. Consequently, the glass substrate of patent document 3, in which the height of convex defects only is specified and neither concave defects nor the sizes of defects are taken into account, cannot be a fully satisfactory substrate for masks for use with an exposure light having a short wavelength, such as EUV light.